1. Field Of The Invention
The present invention relates generally to a process for making paper, and more particularly to the production of wood pulp by the sulfate pulping process for subsequent use in making paper and linerboard.
2. Brief Description Of The Prior Art
In the kraft or sulfate process for making wood pulp carefully dimensioned wood chips are subjected to alkaline reagents (including sulfide ion) at elevated temperature and pressure in a digester. The reaction conditions are carefully chosen to selectively hydrolyze lignin, the amorphous polymeric binder of the wood fibers. The wood fibers are principally composed of cellulose. However, each of the three major constituents of wood, lignin, cellulose and hemicellulose is subject to alkaline hydrolysis and degradation. In carrying out the pulping process, it is desirable to maximize the yield defined as dry weight of pulp per unit dry weight of wood consumed.
Although efforts are usually made to provide wood chips which shape, there are often variations in the size and shape of the wood chips and in the structure and composition of the woods chips themselves. Ideally, the digestion of the wood chips is carried only long enough to dissolve sufficient lignin to free each wood fiber. At this point the digester charge is "blown" into a receiving vessel or blow tank. The sudden drop in pressure serves to mechanically break up the wood chips from which the lignin has been removed. For some paper making applications, it is desirable to subsequently remove the residual lignin, as the lignin confers the characteristic brown color of kraft paper. However, when the object is to produce linerboard or kraft paper it is generally desirable to produce the highest possible yield of wood pulp, although this implies that, in addition to the cellulosic fibers, the pulp will also include non-cellulosic constituents, such as lignin, hemicellulose, natural resins, and other wood constitutents.
While ideally each of the wood chips is completely separated when the digester is blown into the blow tank into separate wood fibers, in practice a fraction of the wood chips fails to separate or only incompletely separates when the digester is blown. These materials are removed from the wood pulp by passing the pulp through a screen having openings of a predetermined size. The materials that are recovered are known as "rejects" in the pulping art. As the rejects include wood fibers, they represent a reduction from the yield of pulp which is ideally achievable. However, the potential yield which these rejects represent cannot be realized simply by lengthening the period of digestion or increasing the severity of the digestion conditions. Although the proportion of rejects would no doubt decline, so also would the total yield because the increased digestion time or more severe hydrolysis conditions would attack not only the lignin in the rejects, but also the cellulose in chips from which the lignin had already been removed.
The digestion of wood chips is a complex process. The chips themselves are highly structured and non-homogeneous. The rate of degradation and removal of amorphous lignin from the chips is believed to be limited by the rate of diffusion of lignin hydrolysis products from the chips. However, this model of the delignification process does not provide insight regarding how to increase its rate. Means to improve the yield in the sulfate pulping process appear to have been found emperically. For example, U.S. Pat. No. 3,909,345, incorporated herein by reference, discloses the use of surface active agents or surfactants having the general formula EQU R[(C.sub.2 H.sub.4 O).sub.m (C.sub.3 H.sub.60).sub.n ]yH
as additives to the sulphate cooking liquor for the purpose of obtaining higher yields of pulp from a given wood chip charge. Chemical Abstracts 94:105141r (W. Surewicz et al, Przegl. Papier. (1980) 36(8)291-4) discloses addition of ethoxylated amines to cooking liquor to increase kraft pulp yield. Chemical Abstracts 104: 1511429 (British Pat. 2,155,966) discloses the addition of an amphoteric surface active agent to aid in the penetration of chemicals into the wood structure and dissolution of the lignin with not more than normal carbohydrate degradation and in a much shorter cooking cycle. The amphoteric surface active agent can be an amidated or quaternized poly(propylene glycol) carboxylate. While some types of surface active agents are disclosed in the art to be useful in increasing penetration of the cooking liquor into wood chips et al., in general, because the process of pulping chips is highly complex and unpredictable, it is not possible to predict whether a particular class of surface active agents will be useful.
The use of nonionic surface active agent in processes for treating wood pulp after the pulp has been prepared by digestion of wood chips is also known. For example, U.S. Pat. Nos. 2,716,058, 2,999,045 and 4,426,254 each relate to the extraction of natural resins from wood pulp. Effective separation of natural resins from the pulp is necessary for the production of purified cellulose as is used in the manufacture of cellophane, viscose rayon, cellulose nitrate, cellulose acetate and like. Of course, use of such surface active agents to solubilize resin would tend to reduce, rather than increase the yield of pulp. Consequently the art which discloses the use of nonionic surface active agents to deresinate wood pulp implicitly cautions against the use of such materials in attempting to increase pulp yield.
There is a continuing need to improve wood pulping processes in general, and kraft pulping processes for the production of linerboard and paper products in particular. Although wood itself is a renewable resource, the continuously increasing demand for linerboard and paper products requires that the most efficient use possible be made of wood as a raw material. Because the kraft pulping of wood chips for linerboard and paper products is carried on such a large industrial scale, processing improvements which yield even small increases in efficiency can have substantial economic and environmental impact.